Thermal properties of perfluorinated n-alkanoic acids self-assembled on native aluminum oxide surfaces

We have examined the thermal and chemical properties of a homologous series of long-chain perfluorinated n-alkanoic acids [CF3(CF2)nCOOH, n=8, 10, 12, 14, 16] self-assembled on native aluminum oxide surfaces using thermal desorption mass spectrometry and monochromatic x-ray photoelectron spectroscopy. These monolayers were prepared by solution self-assembly under ambient conditions on pre-cleaned native aluminum oxide surfaces and then transferred into an ultrahigh vacuum environment for surface analyses. A chemisorption bond strength of ∼30 kcal/mol between the reactive acid head group and the native aluminum oxide surface has been derived from thermal desorption studies. Upon increasing surface temperature, the predominant surface process was found to be molecular desorption. In addition, a minor surface chemical reaction channel also coexists at elevated temperatures, resulting in thermal dissociation of the long chain acid molecules and subsequent formation of a partially fluorinated aluminum oxide surface. These self-assembled molecular monolayers exhibit remarkable long-term ambient stability (∼103 h) which improves progressively with increasing fluorocarbon chain length.